Property or contaminant Units USGS 

parameter code

CAS 

number

Predominantuse group

Common 

MRLs and

LT–MDLs

Human-health benchmark

All data Data from sites used as a source of 

drinking water

Number

of 

samples

Detections exceeding

human-health 

benchmark value

Detections exceeding 

one-tenth of human-health 

benchmark value

Number

of 

samples

Detections

exceeding value

Detections

exceeding one-tenth 

of value

Value Type Number Percent Number Percent Number Percent Number Percent

 Physical properties

Temperature °C 00010 -- -- -- -- -- 640 -- -- -- -- 523 -- -- -- --

Specific conductance μS/cm at 25 °C 00095 -- -- 1 -- -- 639 -- -- -- -- 522 -- -- -- --

Dissolved oxygen mg/L 00300 -- -- 0.1 -- -- 629 -- -- -- -- 516 -- -- -- --

pH Standard units 00400 -- -- -- 6.5 to 8.5 SMCL 639 226 *

-35 *-- -- 522 157 *30 -- --

Alkalinity as CaCO3 mg/L 00410, 00419 or 

39086 or 90410 -- -- 1 -- -- 635 -- -- -- -- 522 -- -- -- --

 Major ions

Bicarbonate mg/L 00440 or 00450 or 

00453 -- -- -- -- 631 -- -- -- -- 518 -- -- -- --

Bromide mg/L 71870 24959–67–9 -- 0.01 -- -- 584 -- -- -- -- 470 -- -- -- --

Calcium mg/L 00915 7440–70–2 -- 0.01–0.02 -- -- 609 -- -- -- -- 492 -- -- -- --

Chloride mg/L 00940 16887–00–6 -- 0.1 250 SMCL 609 0 0.0 -- -- 492 0 0.0 -- --

Fluoride mg/L 00950 16984–48–8 -- 0.1 4 MCL 609 0 0.0 19 3.1 492 0 0.0 16 3.3

Magnesium mg/L 00925 7439–95–4 -- 0.01 -- -- 609 -- -- -- -- 492 -- -- -- --

Potassium mg/L 00935 7440–70–2 -- 0.1 -- -- 609 -- -- -- -- 492 -- -- -- --

Silica mg/L 00955 7631–86–9 -- 0.01 -- -- 609 -- -- -- -- 492 -- -- -- --

Sodium mg/L 00930 7440–23–5 -- 0.2 -- -- 609 -- -- -- -- 492 -- -- -- --

Sulfate mg/L 00945 14808–79–8 -- 0.1 250 SMCL 609 2 0.3 -- -- 492 2 0.4 -- --

Dissolved solids (ROE) mg/L 70300 -- 1–10 500 SMCL 609 18 3.0 -- -- 492 16 3.3 -- --

 Trace elements

Aluminum μg/L 01106 7429–90–5 -- 0.8–1 -- -- 299 -- -- -- -- 217 -- -- -- --

†

Antimony μg/L 01095 7440–36–0 -- 0.05–1 6 MCL 278 1 0.4 4 1.4 196 1 0.5 3 1.5

Arsenic μg/L 01000 7440–38–2 -- 0.2–1 10 MCL 299 9 3.0 52 17 217 9 4.1 48 22

Barium μg/L 01005 7440–39–3 -- 1 2,000 MCL 299 0 0.0 32 11 217 0 0.0 29 13

†Beryllium μg/L 01010 7440–41–7 -- 0.03–1 4 MCL 278 0 0.0 2 0.7 196 0 0.0 0 0.0

Boron μg/L 01020 7440–42–8 -- 7–16 1,000 HBSL 177 0 0.0 0 0.0 136 0 0.0 0 0.0

†Cadmium μg/L 01025 7440–43–9 -- 0.02–1 5 MCL 299 0 0.0 1 0.3 217 0 0.0 0 0.0

Chromium μg/L 01030 7440–47–3 -- 0.4–1 100 MCL 296 0 0.0 0 0.0 214 0 0.0 0 0.0

Cobalt μg/L 01035 7440–48–4 -- 1 -- -- 278 -- -- -- -- 196 -- -- -- --

Copper μg/L 01040 7440–50–8 -- 0.2–1 1,300 Action level 299 0 0.0 1 0.3 217 0 0.0 1 0.5

Iron μg/L 01046 7439–89–6 -- 3–10 -- -- 609 72 12 131 22 492 48 9.8 91 18

Lead μg/L 01049 7439–92–1 -- 0.04–1 15 Action level 299 0 0.0 27 9.0 217 0 0.0 25 12

Lithium μg/L 01130 7439–93–2 -- 0.3–3 -- -- 148 -- -- -- -- 109 -- -- -- --

Manganese μg/L 01056 7439–96–5 -- 0.1–1 300 HBSL 624 37 5.9 162 26 507 22 4.3 111 22

Molybdenum μg/L 01060 7439–98–7 -- 0.03–1 40 HBSL 276 0 0.0 19 6.9 194 0 0.0 17 8.8

Nickel μg/L 01065 7440–02–0 -- 0.2–1 100 HBSL 278 0 0.0 5 1.8 196 0 0.0 1 0.5

Appendix 3. Water-quality properties and constituents measured, a summary of data, and complete data archive for 1993–2008.

Table A3–1. Physical properties and constituents analyzed in samples, 1993–2008, associated human-health benchmarks for drinking water, detection frequency, and frequency of exceeding human-health benchmarks, both in the entire dataset and samples collected from sources of 

drinking water.

[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT–MDLs) are those used for about 10 percent of samples. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public-water supplies; AMCL, USEPA proposed 

Alternative Maximum Contaminant Level for public water supplies; HBSL, health-based screening level developed by the USGS on the basis of USEPA toxicity information and methods (48, 58, 59); ROE, residue on evaporation; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligram per liter; μg/L, microgram per liter; pCi/L, 

picocurie per liter; col/100 mL, number of colonies per 100 milliliters; E. coli, Escherichia coli; --, not available or not applicable]
Table A3–1. Physical properties and constituents analyzed in samples, 1993–2008, associated human-health benchmarks for drinking water, detection frequency, and frequency of exceeding human-health benchmarks, both in the entire dataset, and samples used as a source of 

drinking water—Continued

[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Common method reporting levels (MRLs) and long-term method detection levels (LT–MDLs) are those used for at least about 10 percent of samples. MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public-water supplies; AMCL, USEPA 

proposed Alternative Maximum Contaminant Level for public water supplies; HBSL, health-based screening level developed by the USGS on the basis of USEPA toxicity inform ation and methods (48, 58, 59); °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligram per liter; ROE, residue on evaporation; μg/L, microgram per liter; 

pCi/L, picocuries per liter; col/100 mL, colonies per 100 milliliters; E. coli, Escherichia coli; --, not available or not applicable]

Property or contaminant Units USGS 

parameter code

CAS 

number

Predominantuse group

Common 

MRLs and

LT–MDLs

Human-health benchmark

All data Data from sites used as a source of 

drinking water

Number

of 

samples

Detections exceeding

human-health 

benchmark value

Detections exceeding 

one-tenth of human-health 

benchmark value

Number

of 

samples

Detections

exceeding value

Detections

exceeding one-tenth 

of value

Value Type Number Percent Number Percent Number Percent Number Percent

Selenium μg/L 01145 7782–49–2 -- 0.7–1.2 50 MCL 299 0 0.0 2 0.7 217 0 0.0 2 0.9

Silver μg/L 01075 7440–22–4 -- 0.1–1 100 HBSL 299 0 0.0 0 0.0 217 0 0.0 0 0.0

Strontium μg/L 01080 7440–24–6 -- 10 4,000 HBSL 257 1 0.4 31 12 206 1 0.5 29 14

Thallium μg/L 01057 7440–28–0 -- 0.02–0.9 2 MCL 148 0 0.0 2 1.4 109 0 0.0 2 1.8

Vanadium μg/L 01085 7440–62–2 -- 0.2–1 -- -- 148 -- -- -- -- 109 -- -- -- --

Zinc μg/L 01090 7440–66–6 -- 0.5–1 2,000 HBSL 299 1 0.3 14 4.7 217 0 0.0 10 4.6

 Nutrients and dis solved organic carbon

Nitrite as N mg/L 00613 14797–65–0 -- 0.003–0.01 1 MCL 625 0 0.0 4 0.6 508 0 0.0 3 0.6

Nitrate as N mg/L 00631 *

14797–55–8 -- 0.02–0.05 10 MCL 624 59 9.5 348 56 507 58 11 280 55

Ammonia as N mg/L 00608 7664–41–7 -- 0.02 -- -- 625 -- -- -- -- 508 -- -- -- --

Ammonia plus organic nitrogen as N mg/L 00623 -- -- 0.05–0.2 -- -- 548 -- -- -- -- 438 -- -- -- --

Phosphorus, dissolved as P mg/L 00666 7723–14–0 -- 0.002–0.01 -- -- 486 -- -- -- -- 406 -- -- -- --

Orthophosphate as P mg/L 00671 14265–44–2 -- 0.01 -- -- 623 -- -- -- -- 506 -- -- -- --

Dissolved organic carbon mg/L 00681 -- -- 0.1, 0.3 -- -- 642 -- -- -- -- 520 -- -- -- --

Radon and other radionuclides

Radon pCi/L 82303 14859–67–7 -- 26–80 4,000 pCi/L and 

300 pCi/L

≈ Proposed 

AMCL and 

proposed 

MCL

559 56 10 463 83 478 51 11 395 83

Alpha activity pCi/L 75987 or 63104 12587–46–1 -- 3 15 pCi/L MCL 127 11 8.7 ǂ

-- -- 119 10 8.4 ǂ-- --

Beta activity pCi/L 75989 or 63015 12587–47–2 -- 4 50 pCi/L MCL 127 1 0.8 ǂ

-- -- 119 1 0.8 ǂ-- --

Sum of radium–226 and radium–228 pCi/L Either 09503 or 09511 

plus 81366 7440–14–4 -- Varies by sample 5 pCi/L MCL 128 2 1.6 ǂ

-- -- 120 1 0.8 ǂ-- -- --

Tritium pCi/L 07000 10028–17–8 -- 1 -- -- 169 -- -- -- -- 166 -- -- -- --

Uranium μg/L 22703 7440–61–1 -- 0.4, 1.0 30 μg/L MCL 508 2 0.4 56 11 412 2 0.5 55 13

Bacteria

§

Total coliform §See footnote 31501, 50569, 90900, 

90908 -- -- *

Colony forming unit

per 100 mL §

See footnote MCL 313 187 60 -- -- 313 187 60 -- --

¶

E. coli *

Colony forming unit

per 100 mL

31633, 50278, 50468, 

90901 -- -- *

Colony forming unit

per 100 mL

*

colony forming unit per

100 mL MCL 234 57 24 -- -- 234 57 24 -- --

*

One-tenth of benchmark value not evaluated for non-human-health-related Secondary Maximum Contaminant levels.

† Although concentrations in some samples exceeded one-tenth of the benchmark, is not possible to fully evaluate the detection frequency at one-tenth of the benchmark

because detection limits for a large proportion of the samples were greater than one-tenth of the benchmark.

ǂ Not all radionuclides were not evaluated at one-tenth of the benchmark because of reporting levels.

§ No more than 5 percent of samples were total coliform positive in a month. (For water systems that collect fewer than 40 routine samples per month, no more than one

sample can be total coliform positive per month.) Every sample that has total coliform must be analyzed for either fecal coliform or E. coli if two consecutive TC-positive 

samples, and one is also positive for E. coli fecal coliforms, system has an acute MCL violation.

¶ Fecal coliform and E. coli are bacteria whose presence indicates that the water may be contaminated with human or animal wastes. Disease-causing microbes (pathogens)

in these wastes can cause diarrhea, cramps, nausea, headaches, or other symptoms. These pathogens may pose a special health risk for infants, young children, and people with 

severely compromised immune systems.

≈ U.S. Environmental Protection Agency proposed MCL of 300 used for exceeding one-tenth of human-health benchmark and proposed AMCL of 4,000 used for radon

used for exceeding human-health benchmark.
Table A3–2. Pesticides analyzed in samples, 1993–2008, associated human-health benchmarks for drinking water, detection frequency, and frequency of exceeding human-health benchmarks, both in the entire dataset and in samples collected from sources of drinking water.

[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Predominant-use group is from Gilliom and others(10) and includes compound class for pesticides (AMD, amide; CAB, carbamate; CBE, chlorobenzoic acid esters; CPA, chlorophenoxy acids; DNA, dinitroanalines; MSA, miscellaneous acids; MSC, miscellaneous; 

NPH, nitrophenols; OCL, organochlorines; OPH, organophosphates; PHN, phenols; PYR, pyrethroids; SFE, sulfite esters; TRZ, triazines, URC, uracils; URA, ureas). Pesticide degradation products are designated by use group “DP” with parent compound indicated in parentheses. MCL, U.S. Environmental Protection Agency 

(USEPA) Maximum Contaminant Level for public-water supplies; HBSL, health-based screening level developed by the USGS on the basis of USEPA toxicity information and methods(48, 58, 59), value is low end of range, ass ociated with 10-6 cancer risk; μg/L, microgram per liter; --, not available or not applicable; ≥, greater than or 

equal to]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant-use group

Human-health 

benchmark

Number of samples

All data Data from sites used as a source of drinking water

Value Type

Number of 

detections at any 

concentration

Percentage 

of detections at 

any concentration

Percentage 

of detections 

greater than 

benchmark

Percentage 

of detections 

≥ 0.1 of the 

benchmark

Number of 

samples

Number of 

detections 

at any 

concentration

Percentage 

of detections 

at any 

concentration

Percentage 

of detections 

greater than 

benchmark

2,6-Diethylaniline μg/L 82660 579–66–8 DP (Alachlor) (AMD) -- -- 636 1 0.2 -- -- 522 1 0.2 --

Acetochlor μg/L 49260 34256–82–1 Herbicide (AMD) 1 HBSL 498 1 0.2 0 0 386 1 0.3 0

Alachlor μg/L 46342 15972–60–8 Herbicide (AMD) 2 MCL 637 24 3.8 0 0.2 523 23 4.4 0

alpha-HCH μg/L 34253 319–84–6 DP (gamma-HCH) (OCL) 0.006 HBSL 550 1 0.2 0.2 0.2 441 1 0.2 0.2

Atrazine μg/L 39632 1912–24–9 Herbicide (TRZ) 3 MCL 637 258 41 0 6.1 523 218 42 0

Azinphos-methyl (Guthion) μg/L 82686 86–50–0 Insecticide (OPH) 10 HBSL 636 0 0 0 0 522 0 0 0

Benfluralin μg/L 82673 1861–40–1 Herbicide (DNA) 4 HBSL 636 2 0.3 0 0 523 1 0.2 0

Butylate μg/L 04028 2008–41–5 Herbicide (CAB) 400 HBSL 550 1 0.2 0 0 441 1 0.2 0

Carbaryl μg/L 82680 63–25–2 Insecticide (CAB) 40 HBSL 637 17 2.7 0 0 523 13 2.5 0

Carbofuran μg/L 82674 1563–66–2 Insecticide (CAB) 40 MCL 621 5 0.8 0 0 508 4 0.8 0

Chlorpyrifos μg/L 38933 2921–88–2 Insecticide (OPH) 2 HBSL 636 1 0.2 0 0 522 1 0.2 0

cis-Permethrin μg/L 82687 54774–45–7 Insecticide (PYR) 4 HBSL 636 0 0 0 0 -- -- 0 0

Cyanazine μg/L 04041 21725–46–2 Herbicide (TRZ) 1 HBSL 621 12 1.9 0 0.3 508 10 2.0 0

DCPA (Dacthal) μg/L 82682 1861–32–1 Herbicide (CBE) 70 HBSL 636 1 0.2 0 0 522 1 0.2 0

Deethylatrazine μg/L 04040 6190–65–4 DP (Atrazine) (TRZ) -- -- 637 292 46 -- -- 523 247 47 --

Diazinon μg/L 39572 333–41–5 Insecticide (OPH) 1 HBSL 636 7 1.1 0 0 522 5 1.0 0

Dieldrin μg/L 39381 60–57–1 Insecticide, DP (Aldrin) (OCL) 0.002 HBSL 636 25 3.9 3.6 3.6 522 8 1.5 1.3

Disulfoton μg/L 82677 298–04–4 Insecticide (OPH) 0.9 HBSL 620 0 0 0 0 507 0 0 0

EPTC μg/L 82668 759–94–4 Herbicide (CAB) 200 HBSL 620 3 0.5 0 0 507 3 0.3 0

Ethalfluralin μg/L 82663 55283–68–6 Herbicide (DNA) 30 HBSL 550 0 0 0 0 441 0 0 0

Ethoprop (Ethoprophos) μg/L 82672 13194–48–4 Herbicide (OPH) 1 HBSL 621 0 0 0 0 508 0 0 0

Fonofos μg/L 04095 944–22–9 Insecticide (OPH) 10 HBSL 636 2 0.3 0 0 522 2 0.4 0

gamma-HCH (Lindane) μg/L 39341 58–89–9 Insecticide (OCL) 0.2 MCL 550 1 0.2 0 0.2 441 1 0.2 0

Linuron μg/L 82666 330–55–2 Herbicide (URA) 5 HBSL 565 2 0.4 0 0 456 1 0.2 0

Malathion μg/L 39532 121–75–5 Insecticide (OPH) 50 HBSL 636 0 0 0 0 522 0 0 0

Metolachlor μg/L 39415 51218–45–2 Herbicide (AMD) 700 HBSL 637 149 23 0 0 523 126 24 0

Metribuzin μg/L 82630 21087–64–9 Herbicide (TRZ) 90 HBSL 636 7 1.1 0 0 522 6 1.1 0

Molinate μg/L 82671 2212–67–1 Herbicide (CAB) 0.7 HBSL 620 2 0.3 0 0 507 2 0.4 0

Napropamide μg/L 82684 15299–99–7 Herbicide (AMD) 800 HBSL 551 2 0.4 0 0 442 2 0.4 0

p,p’-DDE μg/L 34653 72–55–9 DP (p,p’-DDT) (OCL) 0.1 HBSL 550 23 4.2 0 0 441 21 4.8 0

Parathion (Ethyl parathion) μg/L 39542 56–38–2 Insecticide (OPH) 0.02 HBSL 550 0 0 0 0 441 0 0 0

Parathion-methyl (Methyl parathion) μg/L 82667 298–00–0 Insecticide (OPH) 1 HBSL 636 0 0 0 0 522 0 0 0

Pebulate μg/L 82669 1114–71–2 Herbicide (CAB) 50 HBSL 550 3 0.5 0 0 441 3 0.7 0

Pendimethalin μg/L 82683 40487–42–1 Herbicide (DNA) 70 HBSL 636 1 0.2 0 0 522 1 0.2 0

Phorate μg/L 82664 298–02–2 Insecticide (OPH) 4 HBSL 636 0 0 0 0 522 0 0 0

Prometon μg/L 04037 1610–18–0 Herbicide (TRZ) 400 HBSL 637 109 17 0 0 523 90 17 0

Pronamide (Propyzamide) μg/L 82676 23950–58–5 Herbicide (AMD) 1 HBSL 637 2 0.3 0 0 523 2 0.4 0

Propachlor μg/L 04024 1918–16–7 Herbicide (AMD) 1 HBSL 551 0 0 0 0 442 0 0 0

Propanil μg/L 82679 709–98–8 Herbicide (AMD) 6 HBSL 621 2 0.3 0 0 508 2 0.4 0

Propargite μg/L 82685 2312–35–8 Acaricide (SFE) 1 HBSL 619 0 0 0 0 507 0 0 0

Simazine μg/L 04035 122–34–9 Herbicide (TRZ) 4 MCL 637 136 21 0 0.2 523 120 23 0

Tebuthiuron μg/L 82670 34014–18–1 Herbicide (URA) 1,000 HBSL 637 32 5.0 0 0 523 23 4.4 0

Terbacil μg/L 82665 5902–51–2 Herbicide (URC) 90 HBSL 565 2 0.4 0 0 456 2 0.4 0

Terbufos μg/L 82675 13071–79–9 Insecticide (OPH) 0.4 HBSL 636 1 0.2 0 0 522 0 0 0

Thiobencarb μg/L 82681 28249–77–6 Herbicide (CAB) 70 HBSL 620 0 0 0 0 507 0 0 0

Triallate μg/L 82678 2303–17–5 Herbicide (CAB) 20 HBSL 550 2 0.4 0 0 441 2 0.5 0

Trifluralin μg/L 82661 1582–09–8 Herbicide (DNA) 20 HBSL 636 5 0.8 0 0 522 3 0.6 0
Table A3–3. Volatile organic compounds analyzed in samples, 1993–2008, associated human-health benchmarks for drinking water, detection frequency, and frequency of exceeding human-health benchmarks, both in the entire dataset, and samples used as a source of drinking water.

[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Predominant use group is from Zorgorski and others.(50) MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public-water supplies; HBSL, health-based screening level developed by USGS using USEPA 

toxicity information and methods(48, 58, 59) ; μg/L, microgram per liter; --, not available or not applicable]

Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use group

Human-health 

benchmark

All data Data from sites used as a source of drinking water

Number of 

samples

*

Number of 

detections at any 

concentration

*

Percentage of 

detections at 

any concentration

Percentage of 

detections 

greater than 

benchmark

Percentage of 

detections greater 

than 0.1 of the 

benchmark

Number of 

samples

*

Number of 

detections at any 

concentration

*

Percentage of 

detections at any 

concentration

Percentage of 

detections greater 

than benchmark

Percentage of 

detections 

greater than 0.1 of 

the benchmark

Value Type

1,1,1,2-Tetrachloroethane μg/L 77562 630–20–6 Solvent 70 HBSL 498 0 0 0 0 385 0 0 0 0

1,1,1-Trichloroethane (TCA) μg/L 34506 71–55–6 Solvent 200 MCL 498 59 12 0 0 385 46 12 0 0

1,1,2,2-Tetrachloroethane μg/L 34516 79–34–5 Solvent 0.3 HBSL 498 0 0 0 0 385 0 0 0 0

1,1,2-Trichloroethane μg/L 34511 79–00–5 Solvent 5 MCL 498 1 0.2 0 0 385 1 0.3 0 0

1,1-Dichloroethane μg/L 34496 75–34–3 Solvent -- -- 498 17 3.4 N/A N/A 385 9 2.3 N/A N/A

1,1-Dichloroethene (-Ethylene) μg/L 34501 75–35–4 Solvent 7 MCL 498 14 2.8 0 0 385 12 3.1 0 0

1,1-Dichloropropene μg/L 77168 563–58–6 Organic synthesis -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

1,2,3,4-Tetramethylbenzene (Prehnitene) μg/L 49999 488–23–3 Gasoline hydrocarbon -- -- 323 1 0.3 N/A N/A 259 1 0.4 N/A N/A

1,2,3,5-Tetramethylbenzene (Isodurene) μg/L 50000 527–53–7 Gasoline hydrocarbon -- -- 323 2 0.6 N/A N/A 259 2 0.8 N/A N/A

1,2,3-Trichlorobenzene μg/L 77613 87–61–6 Organic synthesis -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

1,2,3-Trichloropropane μg/L 77443 96–18–4 Organic synthesis 40 HBSL 498 0 0 0 0 385 0 0 0 0

1,2,3-Trimethylbenzene μg/L 77221 526–73–8 Gasoline hydrocarbon -- -- 323 1 0.3 N/A N/A 259 1 0.4 N/A N/A

1,2,4-Trichlorobenzene μg/L 34551 120–82–1 Solvent 70 MCL 498 1 0.2 0 0 385 1 0.3 0 0

1,2,4-Trimethylbenzene μg/L 77222 95–63–6 Gasoline hydrocarbon -- -- 498 12 2.4 N/A N/A 385 9 2.3 N/A N/A

1,2-Dichlorobenzene (o-) μg/L 34536 95–50–1 Solvent 600 MCL 498 0 0 0 0 385 0 0 0 0

1,2-Dichloroethane (ethylene dichloride) μg/L 32103 107–06–2 Solvent 5 MCL 498 5 1.0 0 0.6 384 2 0.5 0 0.3

1,2-Dichloropropane μg/L 34541 78–87–5 Fumigant 5 MCL 498 6 1.2 0 0.2 385 5 1.3 0 0.3

1,3,5-Trimethylbenzene μg/L 77226 108–67–8 Gasoline hydrocarbon -- -- 498 3 0.6 N/A N/A 385 2 0.5 N/A N/A

1,3-Dichlorobenzene (m-dichlorobenzene) μg/L 34566 541–73–1 Solvent 600 HBSL 498 1 0.2 0 0 385 1 0.3 0 0

1,3-Dichloropropane μg/L 77173 142–28–9 Fumigant -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

1,4-Dichlorobenzene (p-dichlorobenzene) μg/L 34571 106–46–7 Fumigant 75 MCL 498 39 7.8 0 0 385 6 1.6 0 0

2,2-Dichloropropane μg/L 77170 594–20–7 Fumigant -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

2-Chlorotoluene (o-) μg/L 77275 95–49–8 Solvent 100 HBSL 498 0 0 0 0 385 0 0 0 0

2-Ethyltoluene (1-ethyl-2-methylbenzene) μg/L 77220 611–14–3 Gasoline hydrocarbon -- -- 323 1 0.3 N/A N/A 259 1 0.4 N/A N/A

3-Chloropropene (3-chloro-1-propene) μg/L 78109 107–05–1 Organic synthesis -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

4-Chlorotoluene μg/L 77277 106–43–4 Solvent 100 HBSL 498 0 0 0 0 385 0 0 0 0

Acetone (2-propanone) μg/L 81552 67–64–1 Solvent 6,000 HBSL 323 3 0.9 0 0 259 3 1.1 0 0

Acrylonitrile (2-propenenitrile) μg/L 34215 107–13–1 Organic synthesis 0.06 *HBSL 323 1 0.3 0.3 0.3 259 1 0.4 0.4 0.4

Benzene μg/L 34030 71–43–2 Gasoline hydrocarbon 5 MCL 498 17 3.4 0.4 1.2 385 9 2.3 0 0.5

Bromobenzene μg/L 81555 108–86–1 Solvent -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

Bromochloromethane μg/L 77297 74–97–5 Personal care and domestic use product 90 HBSL 497 1 0.2 0 0 384 1 0.3 0 0

Bromodichloromethane (CHBrCl2

) μg/L 32101 75–27–4 Trihalomethane †80 MCL 498 21 4.2 0 0 385 13 3.4 0 0

Bromoform (tribromomethane) μg/L 32104 75–25–2 Trihalomethane †

80 MCL 498 2 0.4 0 0 385 2 0.5 0 0

Bromomethane (methyl bromide) μg/L 34413 74–83–9 Fumigant 100 HBSL 498 0 0 0 0 385 0 0 0 0

Carbon disulfide μg/L 77041 75–15–0 Organic synthesis 700 HBSL 323 16 4.9 0 0 259 15 5.8 0 0

Carbon tetrachloride (tetrachloromethane) μg/L 32102 56–23–5 Solvent 5 MCL 498 6 1.2 0 0.2 385 6 1.6 0 0.3
Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use group

Human-health 

benchmark

All data Data from sites used as a source of drinking water

Number of 

samples

*

Number of 

detections at any 

concentration

*

Percentage of 

detections at 

any concentration

Percentage of 

detections 

greater than 

benchmark

Percentage of 

detections greater 

than 0.1 of the 

benchmark

Number of 

samples

*

Number of 

detections at any 

concentration

*

Percentage of 

detections at any 

concentration

Percentage of 

detections greater 

than benchmark

Percentage of 

detections 

greater than 0.1 of 

the benchmark

Value Type

Chlorobenzene (Monochlorobenzene) μg/L 34301 108–90–7 Solvent 100 MCL 498 1 0.2 0 0 385 0 0 0 0

Chloroethane μg/L 34311 75–00–3 Solvent -- -- 498 2 0.4 N/A N/A 385 1 0.3 N/A N/A

Chloroform (trichloromethane) (TCM) μg/L 32106 67–66–3 Trihalomethane †

80 MCL 498 94 19 0 1.5 385 53 14.0 0 0.5

Chloromethane (methyl chloride) μg/L 34418 74–87–3 Organic synthesis 30 HBSL 498 22 4.4 0 0 385 16 4.2 0 0

cis-1,2-Dichloroethene μg/L 77093 156–59–2 Solvent 70 MCL 497 10 2.0 0 0 385 6 1.6 0 0

cis-1,3-Dichloropropene μg/L 34704 10061–01–5 Fumigant §

0.3 *HBSL 498 0 0 0 0 385 0 0 0 0

Dibromochloromethane μg/L 32105 124–48–1 Trihalomethane †

80 MCL 498 4 0.8 0 0 385 4 1.0 0 0

Dibromochloropropane (1,2-Dibromo3-chloropropane; DBCP) μg/L 82625 96–12–8 Fumigant 0.2 MCL 498 0 0 0 0 385 0 0 0 0

Dibromomethane μg/L 30217 74–95–3 Solvent -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

Dichlorodifluoromethane (CFC-12) μg/L 34668 75–71–8 Refrigerant 1,000 HBSL 483 12 2.5 0 0 370 7 1.9 0 0

Diethyl ether (Ethyl ether; 1,1’-oxybisethane) μg/L 81576 60–29–7 Solvent 1,000 HBSL 323 1 0.3 0 0 259 1 0.4 0 0

Diisopropyl ether (DIPE) μg/L 81577 108–20–3 Gasoline oxygenate -- -- 323 4 1.2 N/A N/A 259 4 1.5 N/A N/A

Ethyl methacrylate μg/L 73570 97–63–2 Organic synthesis -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Ethyl tert-butyl ether (ETBE) μg/L 50004 637–92–3 Gasoline oxygenate -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Ethylbenzene μg/L 34371 100–41–4 Gasoline hydrocarbon 700 MCL 498 4 0.8 0 0 385 2 0.5 0 0

Ethylene dibromide (1,2-dibromoethane; EDB) μg/L 77651 106–93–4 Fumigant 0.05 MCL 498 0 0 0 0 385 0 0 0 0

Hexachlorobutadiene μg/L 39702 87–68–3 Organic synthesis *0.9 HBSL 498 0 0 0 0 385 0 0 0 0

Hexachloroethane μg/L 34396 67–72–1 Solvent 0.7 HBSL 323 0 0 0 0 259 0 0 0 0

Iodomethane μg/L 77424 74–88–4 Organic synthesis -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Isopropylbenzene ((1-methylethyl) benzene) μg/L 77223 98–82–8 Gasoline hydrocarbon 700 HBSL 498 4 0.8 0 0 385 3 0.8 0 0

m- & p-Xylene μg/L 85795 108–38–3 and 

106–42–3 Gasoline hydrocarbon ǂ10,000 MCL 323 1 0.3 0 0 259 1 0.4 0 0

Methyl acrylate (methyl-2-propenoate) μg/L 49991 96–33–3 Organic synthesis -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Methyl acrylonitrile (2-methyl-2-propenitrile) μg/L 81593 126–98–7 Organic synthesis 0.7 HBSL 323 0 0 0 0 259 0 0 0 0

Methyl butyl ketone (n-butyl methyl ketone; 

2-hexanone) μg/L 77103 591–78–6 Solvent -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Methyl ethyl ketone (ethyl methyl 

ketone; MEK) μg/L 81595 78–93–3 Solvent 4,000 HBSL 323 5 1.5 0 0 259 5 1.9 0 0

Methyl isobutyl ketone (isobutyl methyl ketone; 

MIBK) μg/L 78133 108–10–1 Solvent -- -- 323 1 0.3 N/A N/A 259 1 0.4 N/A N/A

Methyl methacrylate μg/L 81597 80–62–6 Organic synthesis 10,000 HBSL 498 0 0 0 0 259 0 0 0 0

Methyl tert-butyl ether (MTBE) μg/L 78032 1634–04–4 Gasoline oxygenate -- -- 497 82 16.5 N/A N/A 385 57 14.8 N/A N/A

Methylene chloride (dichloromethane) μg/L 34423 75–09–2 Solvent 5 MCL 498 14 2.8 0 0.4 385 12 3.1 0 0.3

Naphthalene μg/L 34696 91–20–3 Gasoline hydrocarbon 100 HBSL 498 7 1.4 0 0.2 385 3 0.8 0 0.5

n-Butylbenzene μg/L 77342 104–51–8 Gasoline hydrocarbon -- -- 498 2 0.4 N/A N/A 385 1 0.3 N/A N/A

n-Propylbenzene μg/L 77224 103–65–1 Solvent -- -- 498 3 0.6 N/A N/A 385 2 0.5 N/A N/A

o-Xylene (1,2-Dimethylbenzene) μg/L 77135 95–47–6 Gasoline hydrocarbon ǂ10,000 MCL 323 2 0.6 0 0 259 1 0.4 0 0

Perchloroethene (Tetrachloro-ethene; PCE) μg/L 34475 127–18–4 Solvent 5 MCL 498 68 13.7 0 1.0 385 44 11.4 0 0.5

Table A3–3. Volatile organic compounds analyzed in samples, 1993–2008, associated human-health benchmarks for drinking water, detection frequency, and frequency of exceeding human-health benchmarks, both in the entire dataset, and samples used as a source of drinking water.—Continued

[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Predominant use group is from Zorgorski and others (50). MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public-water supplies; HBSL, health-based screening level developed by USGS using USEPA 

toxicity information and methods( 48, 58, 59); μg/L, microgram per liter; --, not available or not applicable]
Property or contaminant Units

USGS 

parameter 

code

CAS number Predominant use group

Human-health 

benchmark

All data Data from sites used as a source of drinking water

Number of 

samples

*

Number of 

detections at any 

concentration

*

Percentage of 

detections at 

any concentration

Percentage of 

detections 

greater than 

benchmark

Percentage of 

detections greater 

than 0.1 of the 

benchmark

Number of 

samples

*

Number of 

detections at any 

concentration

*

Percentage of 

detections at any 

concentration

Percentage of 

detections greater 

than benchmark

Percentage of 

detections 

greater than 0.1 of 

the benchmark

Value Type

p-Isopropyltoluene (4-Isopropyltoluene) μg/L 77356 99–87–6 Gasoline hydrocarbon -- -- 498 3 0.6 N/A N/A 385 2 0.5 N/A N/A

sec-Butylbenzene μg/L 77350 135–98–8 Gasoline hydrocarbon -- -- 498 2 0.4 N/A N/A 385 1 0.3 N/A N/A

Styrene μg/L 77128 100–42–5 Gasoline hydrocarbon 100 MCL 498 5 1.0 0 0 385 4 1.0 0 0

tert-Amyl methyl ether (Methyl tert-pentyl 

ether; tert-pentyl methyl ether) μg/L 50005 994–05–8 Gasoline oxygenate -- -- 323 5 1.5 N/A N/A 259 4 1.5 N/A N/A

tert-Butylbenzene μg/L 77353 98–06–6 Gasoline hydrocarbon -- -- 498 0 0 N/A N/A 385 0 0 N/A N/A

Tetrahydrofuran (1,4-epoxybutane) μg/L 81607 109–99–9 Solvent -- -- 323 11 3.4 N/A N/A 259 11 4.2 N/A N/A

Toluene μg/L 34010 108–88–3 Gasoline hydrocarbon 1,000 MCL 498 13 2.6 0 0 385 9 2.3 0 0

Total xylene μg/L 81551

108–38–3; 

106–42–3 and 

95–47–6

Gasoline hydrocarbon ǂ10,000 MCL 175 7 4.0 0 0 126 2 1.6 0 0

trans-1,2-Dichloroethene μg/L 34546 156–60–5 Solvent 100 MCL 498 0 0 0 0 385 0 0 0 0

trans-1,3-Dichloropropene μg/L 34699 10061–02–6 Fumigant §

0.3 *HBSL 498 0 0 0 0 385 0 0 0 0

trans-1,4-Dichloro-2-butene μg/L 73547 110–57–6 Organic synthesis -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Trichloroethene (TCE) μg/L 39180 79–01–6 Solvent 5 MCL 498 30 6.0 0 1.4 385 23 6.0 0.3 1.3

Trichlorofluoromethane (CFC-11) μg/L 34488 75–69–4 Refrigerant 2,000 HBSL 483 15 3.1 0 0 370 5 1.4 0 0

Trichlorotrifluoroethane [1,1,2-trichloro-1,2,2-

trifluoroethane] (CFC-113) μg/L 77652 76–13–1 Refrigerant 200,000 HBSL 483 4 0.8 0 0 370 2 0.5 0 0

Vinyl bromide (bromoethene) μg/L 50002 593–60–2 Organic synthesis -- -- 323 0 0 N/A N/A 259 0 0 N/A N/A

Vinyl chloride (chloroethene) μg/L 39175 75–01–4 Organic synthesis 2 MCL 498 0 0 0 0 385 0 0 0 0

* Value is low end of range, associated with 10-6 cancer risk.

† MCL is for sum for four trihalomethanes.

ǂ MCL is for the sum for all xylenes; prior to 1996, samples were analyzed for total xylene; samples collected after 1996 were analyzed for o-xylene and m-& p-xylene.

§ HBSL is for sum of cis-1,3-dichloropropene and trans-1,3-dichloropropene.

Table A3–3. Volatile organic compounds analyzed in samples, 1993–2008, associated human-health benchmarks for drinking water, detection frequency, and frequency of exceeding human-health benchmarks, both in the entire dataset and in samples collected from sources of 

drinking water.—Continued

[USGS, U.S. Geological Survey; CAS, Chemical Abstract Service. Predominant use group is from Zorgorski and others.(50) MCL, U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level for public-water supplies; HBSL, health-based screening level developed by USGS on the basis of 

USEPA toxicity information and methods (48, 58, 59); μg/L, microgram per liter; --, not available or not applicable]